Hard choice. I personally would also go with GForum (not cos im a moderator either ). It has loads of cool features, and also enables "plugins". The template system is great, allowing you to make your own design easily.

Experience with the other 2 has not been particually great. UBB has very messy codes, and IMO was quite hard to add 'hacks' to. vBulletin I never had much luck with installing. It kept on coming up with stuipd little errors that had nothing, or little, to do with the install.

Uh hu....but it WAS the installer scripts problem. This was quite a way back when it was in version 1 (not betas though). Very clumsy if you ask me Anyway, enough about that. Its just my opinion on the matter.

The 3 forums you noted pretty much offer the same end-user functionality and usability. The admin panels and customizability are primarily where you see the differences.

vBulletin ($160) is a good PHP forum that uses MySQL. The admin panel is pretty intuitive and easy to use. It also comes with good documentation. We had a couple of problems with the install, but the developers were extremely helpful in correcting them.

UBB ($199) -- I wouldn't recommend the regular version UBB primarily because as your board gets more active, it gets slower because it uses a flatfile database. It also creates new files for each thread, so if you have a quota for the number of files on your account, this can run into problems. The admin panel (at least for the version we used to use) was intuitive, but they put too many setting options the same page, making it a little cumbersome.

UBBThreads ($229) -- PHP, MySQL based version which is actually the old WWWThreads (Infopop bought out WWWThreads then ceased production and support on the Perl version). Admin panel on the WWWThreads was better than the UBB version. Adding customizations meant working directly with the code. If you're very familiary with Perl, this wasn't difficult (but hard to find where to make the modifications). This comment was directed at the previous WWWThreads, not UBBThreads in Perl or PHP.

Gossamer Forums ($200) -- The admin panel is very clear and every customizable feature is separate from the code. It's a bit of a learning curve to know what variable and templates are responsible for what, but the end result is an easy to maintain and fully customizable solution. The support team is extremely helpful.